Prior art vibration isolation systems are in particular based on pneumatic isolators and may include so-called stage feed-forward. Besides a pneumatic bearing, the isolators comprise position or acceleration sensors. Position sensors are in particular used to detect the height of the anti-vibration mounted load at the location of each isolator.
In stage feed-forward control, acceleration and position information of the displaceable stage are transferred to the controller of a vibration isolation system, which then calculates and outputs control signals for the force actuators of the vibration isolation system based on feed-forward control according to which the forces induced by the displaceable mass are anticipated in six degrees of freedom and compensation forces are provided. Such control systems can be employed independently from and in addition to the actual (feedback and position) control of the vibration isolation system. Occurring forces change depending on the weight, acceleration and displacement of the displaceable stage and therefore determine the compensation forces which are necessary. The compensation forces are usually provided by magnetic actuators.
For many applications (e.g. in the handling of systems concerning the manufacturing of flat panels) the compensation forces which are required have become so large that the use of conventional Lorentz motors (permanent magnets and moving coil system) is no longer economically feasible. Therefore, the trend is to provide the force otherwise.
European patent application EP 2 295 829 A1 (Integrated Dynamics Engineering GmbH) discloses an improved active vibration isolation system which uses the valves of the pneumatic subsystem, i.e. the valves of the pneumatic bearings, to provide compensation forces in a vertical direction. The valves are otherwise only provided for moving the entire system to a specific vertical operating point. However, by appropriate control it is even possible, via changes in pressure, to provide a sufficiently large range of input forces, so that compensation forces in a vertical direction can substantially be provided by the pneumatic subsystem.
In particular for the static forces which in case of a load shift have to be maintained for a rather long time, continuous current supply to the Lorentz motors is now no longer necessary, rather these forces are produced by a pressure increase in the isolators.
In order to provide necessary horizontal compensation forces it is possible to produce a mechanical short circuit to the ground when a force impulse is caused in a horizontal direction, for example by non-Newtonian fluids in the isolators, semi-active, or magnetorheological or electrorheological fluids. This makes it possible to isolate even rather large machines from vibrations of the ground and to minimize the impact of forces induced by displaceable parts on the supported mass, and to minimize temporal effects.
With increasing size of devices to be produced, such as flat panel displays, the floor space required, i.e. the footprint for vibration isolation is increasing as well.
A continuing requirement for the manufacturing process is to allow the panel to be moved over the entire surface by means of displaceable stages, in order to be able to place any portion of the workpiece under specific tools. Hence, the required displacement distance of a displaceable stage is increasing, and thus the support surface for the displacement distance is increasing as well.
So it is hardly possible anymore to provide a sufficiently large uniform support for the displaceable stage, which will not be deformed.
Instead, it is necessary to divide the support into a plurality of sections which are coupled to each other only moderately.
In this case it will be necessary to use more than the typical three or four isolators, not only because of the masses employed. Instead, three or four isolators may be used for each section.
If because of the great mass an appropriately large number of isolators is provided, isolators are connected together to form groups which are provided with control signals from a control loop. For example, four groups may be provided, one at each of the four corners of a support, each group including a plurality of isolators and the isolators of a group are driven in the same manner.
If a stage feed-forward system is employed, it will use position and acceleration signals of the displaceable stage to generate control signals for the valves, which are typically supplied to a so-called pneumatic axis which combines logic combinations of isolator groups and represents physical degrees of freedom (X—rotation, Y—rotation, and Z—translation). However, in principle it is also possible to generate control signals for individual groups of isolators or for individual isolators.
Each of the isolators has a valve for the pneumatic bearing and at least one distance sensor for the vertical direction. In order to combine groups of actuators, an electric line may be installed for each group of valves and controlled by the same control signal.
However, it is not possible by means of the distance sensors to detect a deflection of the structure. Also, a deflection cannot be counteracted when isolators are connected together as actuators across sections.